This invention relates to a moisture vapor permeable, substantially liquid impermeable composite sheet structure useful in apparel, surgical drapes, sterile wraps, packaging materials, protective covers, construction materials, and personal care absorbent articles such as diapers and sanitary napkins. More particularly, the invention is directed to a moisture vapor permeable film and fibrous substrate that combine to form a composite sheet that is durable, strong, and flexible, that acts as a barrier to liquids, bacteria and odors, yet is also highly permeable to moisture vapor. The invention is also directed to an absorbent article having a backsheet made of the aforementioned composite sheet of the invention.
Sheet materials used in making medical drapes, medical gowns and absorbent articles, such as diapers and sanitary napkins, must be both comfortable and substantially liquid impermeable. Manufacturing and use requirements for such products often demand that the sheet material also be strong and durable.
Infants and other incontinent individuals wear absorbent articles to receive and contain urine and other body exudates. Absorbent articles function both to contain the discharged materials and to isolate these materials from the body of the wearer and from the wearer""s garments and bed clothing. Disposable absorbent articles having many different basic designs are known to the art. It is also known that the exterior of absorbent articles can be covered with a flexible, fluid and vapor impervious sheet to prevent any absorbed fluid from passing through the article and soiling adjacent articles such as clothing, bedding and the like. These outer covers, generally referred to as backsheets, are often constructed from fluid and vapor impervious films such as polyethylene.
While plastic films do an admirable job of containing liquids, they are not pleasing to the touch and they do not readily pass moisture vapor, which makes garments made with plastic films uncomfortable and irritating to the skin. Plastic films have been made more acceptable for apparel and personal care applications by creating micropores in the films to make breathable microporous films. In microporous films, moisture is transported through the films by way of small gaps or holes in the film. One notable microporous film composite is made from polytetrafluoroethylene that is adhered to a textile material with an adhesive, as disclosed in British Patent Application No. 2,024,100. Microporous films adhesively bonded to textile substrates have been used in a variety of apparel products, including absorbent articles, as disclosed in PCT Patent Publication Nos. WO 95/16562 and WO 96/39031.
Laminates of a microporous film and a fibrous textile substrate have a number of disadvantages, including that their manufacture requires a separate adhesive bonding step after the film is made, and that such laminates permit some seepage of fluids when used as the backsheet in an absorbent article. For example, when such microporous film laminates are used as a backsheet of a disposable diaper, the backsheet may permit the transmission of some urine through the pores in the backsheet when an infant wearing the diaper sits down. Liquid seepage through microporous film laminates is especially likely to occur when the microporous laminate is exposed to a fluid with a low surface tension, as for example when urine in a diaper is exposed to surfactants within the diaper itself.
When fluids seep through the pores of a microporous film, bacteria, viruses, and other microbes can pass through the film along with the fluids. Likewise, the passage of fluids through laminates made with microporous films, whether the fluids are liquid or gaseous, also increases the odors that emanate from such laminates. Microbial adsorbents have been added to some microporous films in an attempt to capture microbes passing through such films, as disclosed in PCT Patent Publication No. WO 96/39031. However it is difficult to distribute microbial adsorbents throughout a microporous film in a manner that will adsorb all microbes seeping through the holes in the film. Likewise, microbial adsorbents are unlikely to prevent the passage of odors through the pores in a microporous film.
Moisture vapor permeable films comprised of polyether block copolymers, like the film disclosed in U.S. Pat. No. 4,493,870, have an advantage in medical apparel and personal care applications because such films are non-porous and therefore substantially impermeable to fluids, but they permit the passage of moisture vapor. U.S. Pat. No. 4,725,481 suggests that such films may be attached to a textile fabric by adhesive bonding or melt bonding. However, the cost of making such films and then bonding the films to fibrous textile substrates has been high relative to microporous film laminates. In addition, known moisture vapor permeable films like the films disclosed in U.S. Pat. Nos. 4,725,481 and 5,445,874 do not readily adhere to many common nonwoven substrate materials, such as polyolefin-based nonwoven materials, without the application of a separate adhesive.
PCT Patent Publication No. WO 95/16746 (assigned to E. I. duPont de Nemours and Company (hereinafter xe2x80x9cDuPontxe2x80x9d)) discloses a composition of a polyether block copolymer combined with a less costly thermoplastic homopolymer so as to make an overall film that is less costly, more heat sealable and more adherable to itself and other substrate materials. However, PCT Patent Publication No. WO 95/16746 does not disclose strong and durable composite sheets of thin breathable films that have been extruded directly onto fibrous substrates, nor does it disclose a method for making such composite sheets.
There is a need for a sheet material that acts as a barrier to fluids, yet is also highly permeable to moisture vapor. There is also a need for a sheet material that readily transmits moisture vapor, but significantly deters the passage of bacteria and odors associated with such fluids. There is a further need for such a moisture vapor permeable, fluid impermeable composite sheet material that is also durable, strong, and flexible enough to be used in absorbent articles, and can be produced in an economical fashion, i.e., without the use of adhesives to join the layers of the composite sheet in a separate step. Finally, there is a need for an absorbent article that incorporates such a moisture vapor permeable composite sheet in the article""s backsheet, leg cuffs, waistshields, or other features.
The invention provides a moisture vapor permeable, substantially liquid impermeable composite sheet material comprising a fibrous substrate and a moisture vapor permeable thermoplastic film layer. The fibrous substrate is comprised of at least 50% by weight polyolefin polymer fibers. The moisture vapor permeable thermoplastic film layer is melt bonded directly to one side of said fibrous substrate. The composite sheet exhibits a peel strength of at least 0.1 N/cm, a dynamic fluid transmission of less than about 0.75 g/m2 when subjected to an impact energy of about 2400 joules/m2, and a moisture vapor transmission rate, according to the desiccant method, of at least 200 g/m2/24 hr, preferably at least 1500 g/m2/24 hr.
Preferably the film layer of the composite sheet has an average thickness of less than 50 microns and is comprised of at least 50% by weight of polymer selected from the group of block copolyether esters, block copolyether amides, polyurethanes, and combinations thereof. It is further preferred that the film layer be melt bonded to the substrate in the absence of an adhesive between the film layer and the substrate. The more preferred composite sheet has a peel strength of at least 0.15 N/cm, a film thickness of less than 30 microns, and a moisture vapor transmission rate, according to the dessicant method, of at least 2500 g/m2/24 hr, and a dynamic fluid transmission of less than about 0.5 g/m2 when subjected to an impact energy of about 2400 joules/m2. The sheet is also substantially free of micoropores such that substantially no liquid moisture passes through the sheet when tested according to the liquid moisture seepage test, and the sheet acts as a barrier to the passage of microbes when tested according to the ISO 11607 standard for sterile packaging materials. The composite sheet should have a machine direction tensile strength and a cross direction tensile strength of at least 1 N/cm, and a machine direction elongation and a cross direction elongation of at least 30%.
According to one alternative embodiment of the invention the film layer may be bonded between two fibrous substrates. According to another alternative embodiment of the invention, the film layer of the composite sheet may comprise a moisture permeable film having multiple layers, each film layer being comprised of a different moisture vapor permeable thermoplastic polymer composition. One of the multiple layer film layers may comprise a substantially hydrophilic film layer and one of the film layers comprises a substantially hydrophobic film layer. According to yet another embodiment of the invention. the composite sheet may further include an additional layer of diverse construction and composition from the film layer and the fibrous layer, as for example, a microporous film.
According to the preferred embodiment of the invention, the film layer of the composite sheet is comprised at least 50% by weight of a Fraction A consisting essentially of polymer from the group of block copolyether esters, block copolyether amides, polyurethanes and combinations thereof, at least 5% by weight of a Fraction B consisting essentially of a polymer from the group of homopolymers of an alpha-olefin, copolymers or terpolymers containing an alpha-olefin and one or more other monomers, and a block copolymer of a vinylarene and a conjugated diene, and at least 0.1% by weight of a Fraction C consisting essentially of a compatibilizer for Fractions A and B. The film layer preferably comprises, by weight, 50% to 95% film Fraction A, 5% to 40% film Fraction B, and 0.1% to 15% film Fraction C. The film Fraction C preferably consists essentially of homopolymers, copolymers and terpolymers with backbones that are compatible with Fraction B, the backbones being grafted with a monomer having a functional group that is compatible with Fraction A. Film Fraction C is preferably a polymer with a backbone identical to Fraction B, which backbone is grafted with monomer selected from the group of alpha- and beta-ethylenically unsaturated carbonic acids and anhydrides, and derivatives thereof.
The invention also provides a method for making the breathable composite sheet material described above. Thermoplastic polymer selected from the group of block copolyether esters, block copolyether amides, polyurethanes, and combinations thereof is initially mixed. Next, the mixture is simultaneously melted and mixed, and is then melt extruded through a flat film die. The molten mixture is coated directly onto a moving fibrous substrate and is then forced into intimate contact with the fibrous substrate. The molten polymer may be forced into intimate contact with the substrate by passing the polymer coated substrate between cooled nip rollers or by passing the substrate over a vacuum suction inlet. The composite sheet is finally collected on a collection roll.
Finally, the invention provides an absorbent article comprising (a) a topsheet; (b) a backsheet; and (c) an absorbent core located between the topsheet and the backsheet; wherein the backsheet comprises the non-porous, substantially fluid impermeable, moisture vapor permeable composite sheet material described above. Preferably, the composite sheet material is oriented such that the film layer of the composite sheet material faces toward said absorbent core. Where the film layer of the composite sheet comprises a multiple layer film with a substantially hydrophilic elastomer film layer and a substantially hydrophobic elastomer film, the substantially hydrophilic elastomer film is preferably located between the substantially hydrophobic elastomer film and the fibrous substrate. Alternatively, the film layer may further comprise a third film layer comprising a substantially hydrophobic elastomer film located between the substantially hydrophilic elastomer film and the fibrous substrate. The absorbent article may comprise a disposable diaper.